U.S. Pat. No. Re. 24,906 (Ulrich) concerns a pressure-sensitive adhesive tape, the adhesive of which consists essentially of a copolymer of 88-97 parts of alkyl acrylate and correspondingly 12-3 parts of copolymerizable monomer such as acrylic acid. That acrylate copolymer provides excellent adhesion and holding power and experiences no observable deterioration even after years of storage. Although there should be at least four carbon atoms in the alkyl group, or an average of at least four carbon atoms in the alkyl groups, of the alkyl acrylate to provide adequate tackiness at room temperatures, it is recognized that shorter alkyl groups result in acrylate copolymers which become tacky and pressure-sensitive when heated and thus afford the convenience of a pressure-sensitive adhesive tape. Accordingly, the term "pressure-sensitive adhesive" as used here applies to adhesives which may be tack-free at room temperatures but become tacky at elevated temperatures.
To improve certain properties, the acrylate copolymer adhesive as been crosslinked. U.S. Pat. No. 2,973,286 (Ulrich) teaches that solvent resistance is improved by crosslinking by adding an organic peroxide prior to coating and applying somewhat greater heat than would be necessary merely to dry the coating. U.S. Pat. No. 2,925,174 (Stow) teaches crosslinking the acrylate copolymer by reaction with a polyfunctional compound such as a polyamine, a polyol or a polyepoxide. Because crosslinking substantially reduces tackiness, care should be taken to minimize crosslinking until after the tape has been applied to its intended use, and then heating completes the crosslinking.
Known crosslinking agents for acrylate pressure-sensitive adhesives such as those of the aforementioned U.S. Pat. Nos. 2,973,286 and 2,925,174 produce gradual cross-linking during storage at ordinary room temperatures so that unless the crosslinking agent is present in a very small amount, the tape many become useless after a few months. Coatable solutions of the acrylate pressure-sensitive adhesives containing those crosslinking agents usually experience gradually increased viscosity when stored at ordinary room temperatures, so that it has been customary not to add the crosslinking agent to the coating solution until immediately prior to the coating operation.